[yt-dev] Issue #946: Covering grid doesn't set "center" field parameter (yt_analysis/yt)

[Kacper Kowalik]

New issue 946: Covering grid doesn't set "center" field parameter
https://bitbucket.org/yt_analysis/yt/issue/946/covering-grid-doesnt-set-center-field

Kacper Kowalik:

This simple script:


```
#!python

import yt
ds = yt.load("Sedov_3d/sedov_hdf5_chk_0003")
data = ds.covering_grid(1, left_edge=ds.domain_left_edge, dims=[32,32,32])
data["radial_velocity"]
```

yields rather confusing traceback:

```
#!text

Traceback (most recent call last):
  File "bug.py", line 4, in <module>
    data["radial_velocity"]
  File "yt/src/yt-my/yt/data_objects/data_containers.py", line 249, in __getitem__ 
    self.get_data(f)
  File "yt/src/yt-my/yt/data_objects/construction_data_containers.py", line 531, in get_data 
    if len(gen) > 0: self._generate_fields(gen)
  File "yt/src/yt-my/yt/data_objects/data_containers.py", line 719, in _generate_fields
    fd = self._generate_field(field)
  File "yt/src/yt-my/yt/data_objects/data_containers.py", line 286, in _generate_field
    tr = self._generate_fluid_field(field)
  File "yt/src/yt-my/yt/data_objects/data_containers.py", line 306, in _generate_fluid_field
    rv = finfo(gen_obj)
  File "yt/src/yt-my/yt/fields/derived_field.py", line 184, in __call__ 
    dd = self._function(self, data)
  File "yt/src/yt-my/yt/fields/vector_operations.py", line 109, in _radial
    theta = data['index', 'spherical_theta']
  File "yt/src/yt-my/yt/data_objects/data_containers.py", line 249, in __getitem__ 
    self.get_data(f)
  File "yt/src/yt-my/yt/data_objects/construction_data_containers.py", line 531, in get_data 
    if len(gen) > 0: self._generate_fields(gen)
  File "yt/src/yt-my/yt/data_objects/data_containers.py", line 719, in _generate_fields
    fd = self._generate_field(field)
  File "yt/src/yt-my/yt/data_objects/data_containers.py", line 286, in _generate_field
    tr = self._generate_fluid_field(field)
  File "yt/src/yt-my/yt/data_objects/data_containers.py", line 306, in _generate_fluid_field
    rv = finfo(gen_obj)
  File "yt/src/yt-my/yt/fields/derived_field.py", line 184, in __call__ 
    dd = self._function(self, data)
  File "yt/src/yt-my/yt/fields/geometric_fields.py", line 104, in _spherical_theta
    coords = get_periodic_rvec(data)
  File "yt/src/yt-my/yt/fields/field_functions.py", line 51, in get_periodic_rvec
    coords = obtain_rvec(data)
  File "yt/utilities/lib/geometry_utils.pyx", line 89, in yt.utilities.lib.geometry_utils.obtain_rvec (yt/utilities/lib/geometry_utils.c:2228)
TypeError: 'NoneType' object has no attribute '__getitem__'
```

`covering_grid` should either set sane default for *center* or fields using `ValidateParameter()` should catch this earlier and more gracefully.